X-ray Spectra from MHD Simulations of Accreting Black Holes

TL;DR

This study introduces a comprehensive simulation that self-consistently models X-ray spectra from accreting black holes, successfully reproducing observed spectral features and variability across different accretion states.

Contribution

First-principles, self-consistent simulation coupling radiation transport with GRMHD to explain all observed X-ray spectral components of stellar-mass black holes.

Findings

Reproduces thermal peak, power-law, reflection hump, and iron line in spectra.

Shows spectral shape remains constant despite changes in reflection edge.

Finds variability increases with photon energy and inclination, supporting the coronal hot spot model.

Abstract

We present the results of a new global radiation transport code coupled to a general relativistic magneto-hydrodynamic simulation of an accreting, non-rotating black hole. For the first time, we are able to explain from first principles in a self-consistent way all the components seen in the X-ray spectra of stellar-mass black holes, including a thermal peak and all the features associated with strong hard X-ray emission: a power-law extending to high energies, a Compton reflection hump, and a broad iron line. Varying only the mass accretion rate, we are able to reproduce a wide range of X-ray states seen in most galactic black hole sources. The temperature in the corona is T_e ~ 10 keV in a boundary layer near the disk and rises smoothly to T_e >~ 100 keV in low-density regions far above the disk. Even as the disk's reflection edge varies from the horizon out to ~ 6M as the accretion rate decreases, we find that the shape of the Fe K\alpha line is remarkably constant. This is because photons emitted from the plunging region are strongly beamed into the horizon and never reach the observer. We have also carried out a basic timing analysis of the spectra and find that the fractional variability increases with photon energy and viewer inclination angle, consistent with the coronal hot spot model for X-ray fluctuations.